title: Deviation from symmetrically self-similar branching in trees predicts altered hydraulics, mechanics, light interception and metabolic scaling.
abstract: The West, Brown, Enquist (WBE) model derives symmetrically self-similar branching to predict metabolic scaling from hydraulic conductance, K, (a metabolism proxy) and tree mass (or volume, V). The original prediction was Kα V(0.75). We ask whether trees differ from WBE symmetry and if it matters for plant function and scaling. We measure tree branching and model how architecture influences K, V, mechanical stability, light interception and metabolic scaling. We quantified branching architecture by measuring the path fraction, Pf : mean/maximum trunk-to-twig pathlength. WBE symmetry produces the maximum, Pf = 1.0. We explored tree morphospace using a probability-based numerical model constrained only by biomechanical principles. Real tree Pf ranged from 0.930 (nearly symmetric) to 0.357 (very asymmetric). At each modeled tree size, a reduction in Pf led to: increased K; decreased V; increased mechanical stability; and decreased light absorption. When Pf was ontogenetically constant, strong asymmetry only slightly steepened metabolic scaling. The Pf ontogeny of real trees, however, was 'U' shaped, resulting in size-dependent metabolic scaling that exceeded 0.75 in small trees before falling below 0.65. Architectural diversity appears to matter considerably for whole-tree hydraulics, mechanics, photosynthesis and potentially metabolic scaling. Optimal architectures likely exist that maximize carbon gain per structural investment.
authors.affiliation: Department of Biology, University of Utah, Salt Lake City, UT, 84112, USA.
authors.forename: Duncan D
authors.initials: DD
authors.lastname: Smith
authors.affiliation: Department of Biology, University of Utah, Salt Lake City, UT, 84112, USA.
authors.forename: John S
authors.initials: JS
authors.lastname: Sperry
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, 85721, USA.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Enquist
authors.affiliation: Department of Biomathematics, University of California, Los Angeles, CA, 90095, USA.
authors.forename: Van M
authors.initials: VM
authors.lastname: Savage
authors.affiliation: Department of Forest Ecosystems and Society, Oregon State University, Corvallis, OR , 97331, USA.|Department of Botany, University of Wisconsin-Madison, Madison, WI, 53706, USA.
authors.forename: Katherine A
authors.initials: KA
authors.lastname: McCulloh
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, 85721, USA.
authors.forename: Lisa P
authors.initials: LP
authors.lastname: Bentley
mesh_terms: D001696:Biomechanical Phenomena; D008027:Light; D008954:Models, Biological; D010788:Photosynthesis; D018515:Plant Leaves; D018547:Plant Stems / Q000254:growth & development*; D018526:Plant Transpiration; D014197:Trees / Q000033:anatomy & histology / Q000254:growth & development / Q000378:metabolism / Q000502:physiology*; D014867:Water / Q000502:physiology*
chemical_list: D014867:Water
keywords: Euler buckling; West Brown and Enquist; branching symmetry; hydraulic architecture; light interception; metabolic scaling theory; plant allometry